<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        ul{
            list-style: none;
            cursor: pointer;
        }
        .uu {
            margin: 100px auto;
            display: flex;
            width: 340px;
            height: 30px;
            justify-content: space-evenly;
            background: url("images/bg.jpg");
        }
        .uu div,
        .next li{
            width: 100px;
            height: 30px;
            background: url("images/libg.jpg");
            line-height: 30px;
            text-align: center;
        }
        .next{
           display: none;
        }

    </style>
</head>
<body>
<ul class="uu">
    <li>
        <div>一级菜单1</div>
        <ul class="next">
            <li>二级菜单11</li>
            <li>二级菜单12</li>
            <li>二级菜单13</li>
        </ul>
    </li>
    <li>
        <div>一级菜单2</div>
        <ul class="next">
            <li>二级菜单21</li>
            <li>二级菜单22</li>
            <li>二级菜单23</li>
        </ul>
    </li>
    <li>
        <div>一级菜单3</div>
        <ul class="next">
            <li>二级菜单31</li>
            <li>二级菜单32</li>
            <li>二级菜单33</li>
        </ul>
    </li>
</ul>

<script src="jquery-3.4.1.min.js"></script>
<script>

    $(".uu li").mouseenter(function () {
        $(this).children(".next").stop().show(200);
    }).mouseleave(function () {
        $(this).children(".next").stop().hide(200);
    });
</script>
</body>
</html>